advertisment Advertisement
This ad will close in  seconds. Skip now
advertisment Advertisement
advertisment Advertisement
advertisment Advertisement
advertisment Advertisement
Industry News / Subsystems and Systems

Radio On a Chip for Affordable Wireless Networking

September 1, 2003
/ Print / Reprints /
| Share More
/ Text Size+

A new Radio On a Chip (ROC) has been developed to meet the needs of the emerging short range and low data rate wireless link market. The HRF-ROC093XC ROC has been introduced for wireless, digital data applications in a broad arena of industries. This radio operates in the 902 to 928 MHz ISM band providing up to 128 kbps for low cost, private, wireless networking systems.


This transceiver was developed to lower the cost of migration to wireless capability in a number of different ways. First, the transceiver itself is low cost and reduces the quantity of components required to implement a radio. Second, the high level of integrated digital functionality allows use of a lower cost microcontroller and less software development. Finally, Honeywell can provide flexible frequency hopping software that provides a jump-start to the overall software development task.

The HRF-ROC093XC ROC is an all CMOS, 2.5 V, programmable transceiver fabricated on a 0.25 µm process. The transceiver includes RF signal synthesis, a frequency shift keying (FSK) transmitter, a direct down-conversion receiver and a feature-rich digital interface. Designed to operate under license-free FCC Part 15.247 and 15.249, the chip can be used as a single frequency radio for short-range applications and as a frequency hopping spread spectrum (FHSS) radio for longer-range devices. Frequency hopping provides the benefits of using higher transmit power (longer range) and multiple channels to nearly eliminate interference from other radios. A block diagram of the ROC is shown in Figure 1 . Table 1 lists key performance specifications and features.

Table 1
Key Specifications and Features

Operating frequency (MHz)

860 to 928

Data rate (kbps)

128 max.

Rx sensitivity (dBm)

-95 to -100

Transmitter power (dBm)

+6 max.

Integrated RF switch

yes

On-chip data storage

64 byte FIFO

Programmable address/correlator

32 bits

FHSS capable

yes

DC current (Tx mode) (mA)

30

DC current (Rx mode) (mA)

37

Integrated Digital Functions and Microcontroller Interface

In order to reduce costs and make it easy to insert this transceiver into embedded applications, several features were added to the device. It starts with a standard microcontroller interface, the Serial Peripheral Interface (SPI) bus. This is the 6 MHz pipeline for control and data transfer between the ROC and the microcontroller.

Through the SPI, the user has great control over the operation through the transceiver registers. Control functions include transceiver operating mode (Tx/Rx/Sleep), RF operating frequency, transmitter power and PLL reference frequency. Also controlled is the data rate, Manchester encode/decode, message address length and value, digital sampling, and digital correlator threshold.

In many systems, the software must perform many wireless communication functions. This can include formatting of messages, encoding and decoding data, and determining if the message was meant for that device. Since the data is usually in the 10 to 100 kbps range, this is a very inefficient use of processor resources. The HRF-ROC093XC has integrated these functions to greatly improve efficiency. After writing the data into the FIFO, the chip performs all the packet formatting and transmission functions. The ROC will Manchester encode the data, attach a programmable 32-bit address, append a preamble and send the data to the modulator.

RF Synthesizer and Transmitter

The on-chip VCO, PLL and crystal circuits operate as the RF signal synthesizer. Implemented as a common "divide by N" style PLL, the frequency can be controlled by simply writing to a register via the SPI bus. The typical application starts with closed loop operation using an 8 kHz loop filter with a 100 kHz reference frequency. This provides an RF signal, as shown in Figure 2 . Supporting frequency hopping applications, the frequency can be controlled to operate over the 902 to 928 MHz band by writing to a single register. The crystal reference frequency is in the 1 to 16 MHz range. Channel switch time is less than 400µs and modulation deviation is 175 kHz (high data rate).

Table 2
Receiver Characteristics

Receiver

Sensitivity (dBm)
Dynamic range (dB)

-95 to -100
105 to 110

LNA

Gain (dB)
NF (dB)

25
<3

Mixer

Gain (dB)
NF (dB)

-12
12

Baseband Filter

High pass (kHz)
Cutoff frequency (kHz)
Attenuation (dB)

50
275 adjustable
60 at 1 MHz (high data rate)

Analog and Digital Receiver

The RF and analog section of the receiver is a direct down-conversion architecture, including the FSK demodulator. Referring to the block diagram, the receiver includes an LNA, mixer, baseband filters, limiter and demodulator.

The LNA is a multi-stage amplifier with both single-ended and differential amplifiers. Adjustable currents provide the ability to trade off gain and power dissipation to tailor the system to long- or short-range applications.

The direct downconversion is accomplished using dual mixers for IQ baseband representation. External resistors allow the mixer LO drive to be controlled to optimize the mixer conversion gain versus intercept point. Active filtering follows the mixer and consists of filters with a high frequency cutoff of 275 kHz.

The limiter and demodulator provide remarkable performance in the presence of small and large RF input signals contributing to the high dynamic range of the receiver. Input signals as high as +10 dBm are still received without degrading the bit error rate (BER). Following the signal detection, an adjustable hysterisis window is provided for high system sensitivity or squelch protection against in-band interference. A summary of technical characteristics of the receiver is provided in Table 2 .

The received demodulated data now enters the digital receiver section of the ROC. It is here the transceiver simplifies software design and reduces the microcontroller workload. The digital filter over-samples and applies hysterisis to the data to remove glitches, spurious noise and account for jitter.

A unique feature of the ROC is the detection of the message address with the digital correlator function. Only if the in-coming message address matches the correlator will data be allowed to enter the FIFO. A block diagram of the correlator is shown in Figure 3 . The programmable features include length (up to 32 bits are selected), value (the correlation pattern is selectable), mask (the actual bits to be correlated are selected and bits can be masked off) and threshold (defines the number of matching bits between the data in the correlator and the pattern in the correlator coefficient register). As the serial data begins to enter the correlator, it will compare data bits with the correlator value with the mask in place. Once enough bits are matched to reach the threshold, the data will then be sent into the FIFO.

ROC Frequency Hopping Software

The ROC requires software to perform the media access protocol associated with frequency hopping. Software written in C has been developed, which is flexible and will support several communication system architectures including base station/remote device, broadcast, point to point and point to multi-point. The software does not constrain developers to one type of media access protocol or number of devices in the network.

The functions performed by the software establish initial communication, reliable message transport and re-synchronization. To establish initial communication, the algorithm will send a "sync message" between devices and synchronize them so they are on the same channel at the same time. Transfer of timing messages continuously maintains synchronization. To ensure reliable message transport, the protocol incorporates an ACK and re-send algorithm including "re-send if the message is not received." In re-synchronization, if the RF link is broken and then made available again, software will automatically re-synchronize and continue sending data. It also continues data transmission at the point where it was when link was broken.

Applications and Product Development

The application space for the ROC is quite broad and encompasses some traditional markets and some new areas. Traditional applications are the lower data rate ones for remote sensors, real-time remote industrial metering and home control. The higher data rate HRF-ROC093XC extends the application space beyond to handheld games, digital voice communications, image transfer, wireless modems, personalized convenience store key chains and multiple node data networks. With minimal additional infrastructure, these radios can be used for networks that support larger areas like industrial building, farms and marinas.

A typical network would include several remote wireless devices communicating with a base station. This base station would then be connected back to a main server/facility that collects data or controls the remote devices. In this instance a wireless modem is created. The individual remote devices can also communicate with each other for control or extended range.

Comparison to Emerging Standards

This radio was designed for simple, low cost, private networks requiring low to medium data rates. When compared to other radio systems on the market, such as Bluetooth, the ROC provides longer range, lower power, lower cost and less memory space for protocol.

Many sensor, modem and control applications do not require the high data rates (up to 1 Mbps) and complex multi-tier networking supported by Bluetooth. Therefore, ROC can be implemented easily into various wireless communication protocols that can be tailored to meet the unique needs of each application.

When looking at ZigBee/IEEE 802.15.4 radios, which expect to start showing up in 2004, several differences exist. The ROC supports data rates up to a maximum of 128 kbps in the 915 MHz band and ZigBee/IEEE 802.15.4 is at 40 kbps. The ROC is frequency hopping spread spectrum (FHSS) and the ZigBee/IEEE 802.15.4 is direct sequence spread spectrum (DSSS). In addition, ZigBee will license the networking and media access protocol with the 802.15.4 radio. This may or may not suit the applications requiring higher data throughput. The ROC allows the user to implement its own protocol with peak efficiency to that specific application.

There are a large number of applications in which the higher data rates and ability to supply a customized protocol would yield the best product. In these areas, ROC would be the best fit.

Honeywell, Solid State Electronics Center, Plymouth, MN (800) 323-8295, www.mysoiservices.com. Circle No. 303

Post a comment to this article

Sign-In

Forgot your password?

No Account? Sign Up!

Get access to premium content and e-newsletters by registering on the web site.  You can also subscribe to Microwave Journal magazine.

Sign-Up

advertisment Advertisement